Someone who takes a long time to get something...i.e. he/she is slow. The word's short form is tubie...it literally means a florescent bulb... because when you switch it on the light doesnt actually flash until a few seconds later. Similarly, the person's brain whom you called a tubelight, doesn't light up and understand something until later

Rose is such a tubelight, she didn't get the joke until after everyone stopped laughing